This week 19 of the 38 emergency cross-passages linking the 16km northbound and southbound tunnels under the Chilterns have been mined. 

The two main tunnels are being excavated by two Herrenknecht TBMs, named Florence and Cecilia.

To build the cross-passages, a team of miners is using excavators to break out of the southbound running tunnel and dig the 15-20m to the adjacent northbound tunnel. As the excavation progresses the ground is supported using a sprayed concrete lining.

HS2 Ltd project manager Aaron Harrison said: “We’re making great progress on the Chiltern tunnel with approaching 9 out of 10 miles complete, and our cross-passage teams are catching up fast. While invisible to the travelling public, these hidden passages have a key role in providing a safe operational railway so it’s great to see such good progress.”

The Chiltern tunnel is being delivered by HS2’s main works contractor, Align – a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick, which is also responsible for the nearby Colne Valley Viaduct, which also reached the halfway point last week.

Align’s underground construction director, Didier Jacques, said he was delighted with the progress made by the cross-passage team, working in co-operation with supply chain partners.

“In particular I would like to acknowledge the contribution from KVJV who is responsible for the ground treatment, specialist contractor Euro Diamond Drilling and waterproofing specialist Renesco,” he said.

Once the cross-passages are complete, they are temporarily sealed for fire safety reasons before permanent safety doors can be installed at either end. The doors are being manufactured by Bolton-based Booth Industries which won the contract to provide more than 300 units for all the tunnels between London and Birmingham.

The sliding doors have been subjected to rigorous fire and fatigue testing to take account of the high train speeds, associated pressures and frequency of the trains. 

Each running tunnel has an emergency walkway that allows passengers to evacuate the train and walk through the cross-passages into the other tunnel where they can be rescued on a passenger train. Access for emergency services is from either the portal or one of the five ventilation/emergency access shafts.

An additional five adits connecting the shafts to the running tunnels, are also under construction by the team. The mining on two of the adits is complete.  

TBMs Florence and Cecilia are expected to breakthrough at the north portal early next year.